1. Field of the Invention
The present invention relates to the manufacture in a high speed rod rolling mill, of steel concrete-reinforcing rod, which can have both a high limit of elasticity and high ductility, and, if desired, good weldability; the manufacturing process involves a short cooling process applied during or immediately after rolling.
2. Description of the Prior Art
If the mill operator wishes to resolve the problem which has just been posed, he must take into account the various constraints imposed on him. In the first place, his rolling installation, in practice, determines the delivery speed and temperature of the rod; furthermore, the mill operator has limited space for the possible installation of a cooling device.
Particularly in the case of rod rolling mills, the delivery speed in modern installations is very high, of the order of a hundred meters per second.
From a metallurgical point of view, there are already several solutions available for reaching a compromise between the mechanical properties on the one hand and the cost price on the other hand.
A first solution consists in producing "naturally hard" steel reinforcements whose limit of elasticity is obtained by adding carbon (for example 0.35 wt. %) and manganese (for example 1.3 wt. %); these steels have an acceptable limit of elasticity (420 MPa), but their elongation and their aptitude for bending are relatively low and their weldability clearly insufficient.
In order to improve the weldability, it is necessary to decrease the carbon content, which results in a decrease in the limit of elasticity.
There are two known methods for compensating this decrease in the limit of elasticity.
The first consists in incorporating microalloying elements, such as niobium or vanadium in the steel. This technique is costly however, owing to the price of the alloying elements.
The second method is to increase the limit of elasticity of the steel, by means of cold deformation of the bar, in particular by twisting. Apart from the costs also incurred by an operation of this kind, the gain in the limit of elasticity is produced to the detriment of the elongation.
The method with which the present invention is concerned ranks among recent technology which consists in applying a short cooling process, which is limited in time, to the hot-rolled concrete-reinforcements, during or immediately after rolling, so as to produce a surface layer of martensite in the bar; this "quenching" is followed by a cooling process during which the core of the bar, i.e. the part not affected by the short cooling process, is transformed into ferrite and carbides. By carefully limiting the duration of the short cooling process, it is further possible to preserve the heat in the core of the bar and to produce in its cross-section a temperature gradient such that, during the said subsequent cooling process, tempering of the surface layer of martensite is produced. The duration of the short cooling process can be carefully limited in this way by ensuring that a determined core temperature, at the end of the short cooling phase, is achieved; in practice, such an operation can be carried out by observing the surface temperature at the point on the bar at which reheating, as a result of the supply of heat coming from the core, is observed.
A method of this type, commonly called "quenching and self-tempering", can in theory be carried out--in an installation determined in accordance with the known specifications for manufacturing specific reinforcements--on the basis of the feature of the "core" temperature at the end of the short cooling phase.
However, it is understood that implementing the operation has various difficulties depending on the speed of advance of the products on the one hand and their diameter on the other hand.
At the present stage, although the manufacture of reinforcements of this type no longer poses any difficulty when processing bars having a minimum diameter of 6 mm, the case is different on rod rolling mills operating at high speed; in an installation of this type, the use of intense water cooling devices actually produces disturbing effects with respect to the displacement of the product.